A photoplethysmographic (PPG) signal can be measured by PPG systems to derive corresponding physiological signals (e.g., pulse rate). In a basic form, PPG systems can employ a light source or light emitter that injects light into the user's tissue and a light detector to receive light that reflects and/or scatters and exits the tissue. The received light includes light with an amplitude that is modulated as a result of pulsatile blood flow (i.e., “signal”) and parasitic, non-signal light with an amplitude that can be modulated (i.e., “noise” or “artifacts”) and/or unmodulated (i.e., DC). Noise can be introduced by, for example tilt and/or pull of the device relative to the user's tissue, hair, and/or motion.
For a given light emitter and light detector, the PPG pulsatile signal (i.e., detected light modulated by pulsatile blood flow) can decrease as the separation distance between the light emitter and light detector increases. On the other hand, perfusion index (i.e., the ratio of pulsatile signal amplitude versus DC light amplitude) can increase as the separation distance between the light emitter and light detector increases. Higher perfusion index tends to result in better rejection of noise due to motion (i.e., rejection of motion artifacts). Therefore, shorter separation distances between a light emitter and a light sensor can favor high PPG signal strength, while longer separation distances can favor high perfusion index (e.g., motion performance). That is, a trade-off can exist, making it difficult to optimize separation distance for particular user skin/tissue types and usage conditions.
Additionally, the PPG system can include several light emitters, light detectors, components, and associated wiring that may be visible to a user's eye, making the PPG system aesthetically unappealing.